The mechanism behind the formation of dye images in a silver halide color photographic material is that an aromatic primary amine developing agent, while reducing silver halide grains in the exposed photographic material, is oxidized and the resulting oxidized product reacts with a coupler already present in the silver halide color photographic material so as to form a dye. Color reproduction in this case depends commonly on the subtractive process using three couplers which respectively form yellow, magenta and cyan dyes. These couplers are added to silver halide emulsion layers after they are dissolved in a substantially water-soluble high-boiling organic solvent, optionally in combination with an auxiliary solvent.
There are several requirements that must be met by the couplers: first, they must have high solubility in high-boiling organic solvents, and they should be highly dispersible in silver halide emulsions and the prepared dispersion should remain stable without causing the precipitation of the couplers; secondly, the couplers should have sufficiently good spectral absorption characteristics and color tone to produce sharp dye images over a broad color reproduction range; and thirdly, the couplers should produce dye images which are fast to light, heat and moisture.
Among the three dye-forming couplers, the cyan coupler is required to provide a cyan dye image having sufficient resistance to light, heat and moisture so that it can be stored in a well balanced manner in terms of deterioration resulting from light, heat and moisture. A particularly important requirement is that the cyan dye image has improved dark discoloration when exposed to heat and moisture.
Illustrative cyan couplers that satisfy these requirements are 2,5-diacylaminophenols having an acylamino group as a substituent on the 2- and 5-positions of the phenol ring, and cyan couplers of this type are shown in U.S. Pat. No. 2,895,826, as well as Japanese Unexamined Published Patent Application Nos. 112038/1975, 109630/1978 and 163537/1980.
Such 2,5-diacylaminophenolic cyan couplers produce cyan dye images having improved keeping quality, particularly in terms of dark discoloration. Additionally, the image has high stability in a processing solution, especially a bleach-fixing solution, and exhibits good recoloring properties.
However, these cyan couplers have the following disadvantages: (1) they have a short wavelength for peak spectral reflection and a high minimum spectral density (hence, low brightness) in the region of 450-480 nm, thereby providing a small color reproduction range and, additionally, the high absorption in the 500-550 nm range causes poor reproduction of the green color; (2) the couplers do not have adequate resistance to light; and (3) the couplers are low in dissolvability and dispersion stability. Furthermore, the sensitivity of silver halides in a coating solution of a silver halide photographic emulsion that contains such 2,5-diacylaminophenolic cyan couplers (hereunder simply referred to as a coating solution) and which is immediately applied to the substrate after its preparation differs greatly from the sensitivity obtained when the coating solution is allowed to stand for a certain period after its preparation. This shortness of the period during which the sensitivity of silver halide grains remains the same has heretofore prevented the mass production of silver halide photographic materials having consistent quality.
The problem of the sensitivity variation following the preparation of an emulsion coating solution can be effectively solved by adding a sensitizing dye to the coating solution, but as more sensitizing dye is added, frequent dye staining occurs.
Other methods have been proposed for extending the period during which the silver halide sensitivity retains its initial high level: they include (1) addition of a known stabilizer (e.g. azoles or azaindene compounds) to the coating solution; (2) adding a reducing agent (e.g. hydroquinones or sulfinic acids) to the coating solution; and (3) using a specific copolymer in combination with a brightener, as described in Japanese Unexamined Published Patent Application No. 111629/1974. However, none of these methods are capable of attaining the intended object in a completely satisfactory manner, and some of them even cause adverse effects on important photographic properties such as tone gradation and sensitivity.